This invention relates to a method of detecting abnormality in an evaporative fuel-purging system for an internal combustion engines.
An evaporative fuel-purging system, which is also called "evaporative emission control system", comprises a canister for temporarily storing evaporative fuel from a fuel tank, and purging control means for controlling purging of the evaporative fuel into the intake system of the engine when the engine is operating.
An evaporative fuel-purging system of this kind can undergo deterioration of the canister, disengagement of joints of the piping, etc., which results in improper purging of the evaporative fuel. Therefore, it is waited for to provide a method of detecting such failure.
Conventionally, a fuel supply control system for an internal combustion engine is known, e.g. from Japanese Provisional Patent Publication (Kokai) No. 63-186955, in which an air-fuel ratio feedback control correction coefficient is determined based on an air-fuel ratio signal from an air-fuel ratio sensor for controlling an amount of fuel supplied to the engine, and at the same time evaporative fuel from the fuel tank is purged into the intake passage. In this known fuel supply control system, the evaporative fuel is supplied to the intake passage at a location at which negative pressure prevails when a throttle valve in the intake passage is opened by a predetermined degree or more from a fully closed position thereof. Therefore, an amount of evaporative fuel supplied to the intake passage assumes a value substantially equal to 0 when the engine is idling, and the maximum value when the engine is in a low load condition. Based on the recognition of this phenomenon, a system has been proposed in the above-mentioned publication, which is adapted to calculate the difference between a central value of the above-mentioned correction coefficient obtained during idling of the engine and a central value of same obtained when the engine is in a low load condition, and estimate from the difference the concentration of evaporative fuel corresponding to an amount of evaporative fuel which is actually purged into the intake passage.
Therefore, it is possible to detect whether or not there is failure in the evaporative fuel-purging system by utilizing the above-mentioned manner of estimating the concentration of evaporative fuel, i.e. by comparing an estimated actual value of the concentration of evaporative fuel, i.e. the amount of evaporative fuel actually purged into the intake passage with a reference value of the amount of evaporative fuel purged into the intake passage, which should be obtained when the evaporative fuel-purging system is normally functioning under the same conditions as the estimated actual value is obtained.
However, when evaporative fuel is purged into the intake passage, generally the amount of evaporative fuel purged largely fluctuates depending on a change in the magnitude of load on the engine, particularly a change in the degree of opening of the throttle valve, and accordingly, the air-fuel ratio correction coefficient largely varies under the influence of fluctuations in the amount of evaporative fuel purged, i.e., depending on the load on the engine. Particularly when the engine is in a low load condition, the variation in the correction coefficient is large.
Therefore, in the above proposed system, the central value of the correction coefficient obtained when the engine is in a low load condition fluctuates with a change in the magnitude of load on the engine, so that it is difficult to obtain a stable and accurate central value. This in turn makes it difficult to obtain an accurate estimated value, and accordingly the use of an inaccurate estimated value makes it impossible to accurately detect failure in the evaporative fuel-purging system.
Further, the estimated value of the amount of evaporative fuel purged into the intake system varies depending on an amount of evaporative fuel actually stored in the canister. More specifically, if the amount of evaporative fuel stored in the canister is small, the amount of evaporative fuel purged under a low load condition of the engine is small. Therefore, the amount of change in the correction coefficient between the idling and the low load condition of the engine is small in such a case. This may bring about an erroneous detection of failure in the evaporative fuel-purging system. This also makes it difficult to accurately detect failure of the evaporative fuel-purging system by utilizing the manner of estimating the concentration of evaporative fuel disclosed in the aforementioned publication.